1. Field of the Invention
This invention relates to a shift control system for an automatic transmission with a torque converter.
Automatic transmissions for automotive vehicles, which generally have torque converters and transmission gear mechanisms, typically include pluralities of frictional coupling elements, such as clutches and brakes. In such an automatic transmission, the frictional coupling elements are selectively locked and unlocked so as to change a torque transmission path of the transmission gear mechanism, thereby shifting the automatic transmission to any desired gear. In order to selectively lock and unlock the frictional coupling elements, the automatic transmission is provided with a hydraulic control system.
2. Description of Related Art
A transmission gear mechanism of an automatic transmission of this kind conventionally includes a gear mechanism, such as a planetary gearset having a plurality of gears and a carrier. A hydraulic control system, which manages selective locking and unlocking of the frictional coupling elements of the transmission gear mechanism, includes pressure regulating valves for adjusting pressure provided by an oil pump to a predetermined line pressure level, a manually operated valve for shifting the automatic transmission into various ranges, and a plurality of shift valves automatically operated according to engine operating conditions for selectively opening and closing hydraulic oil paths so as to selectively lock and unlock the frictional coupling elements. The hydraulic control system further includes solenoid valves controlled by a control unit so as to actuate the shift valves.
In this kind of automatic transmission, if necessary frictional coupling elements are locked too early or unlocked too late during a shift, an engine equipped with the automatic transmission is subjected to what is known as a "speed decline" which results in a drop in output torque. However, if the necessary frictional coupling elements are locked too late or unlocked too early, the engine is subjected to what is known as a "speed boost," which results in an abrupt increase in output torque. In order to eliminate such a drop or rapid increase in output torque, the frictional coupling elements must be locked and unlocked at well controlled timings.
For locking and unlocking the frictional coupling elements of the automatic transmission at appropriate timings, the hydraulic control system is provided with orifices and bypass passages for bypassing the orifices. The orifices and bypass passages are provided in hydraulic passages for supplying hydraulic pressure to servo mechanisms. The bypass passages are opened and closed by timing valves actuated and deactuated by solenoid valves. Specifically, such a timing valve is actuated so as to open the bypass passage for a desired period of time at the beginning of a shift operation of the automatic transmission, thereby regulating a timing at which necessary frictional coupling elements are locked and unlocked. Subsequently, the bypass passage is closed so that the hydraulic pressure is supplied through the orifice for a gentle shift operation.
During, for instance, a 3--2 (third gear to second gear) down-shift of the automatic transmission, it is typical to release hydraulic pressure supplied to the servo mechanism so as to lock a 2--4 brake simultaneously with unlocking of a 3--4 clutch. In order to reduce shift shock caused in the automatic transmission during a gear shift, it is necessary to change a locking timing for the 2--4 brake in correspondance with vehicle speeds. For this reason, some of the conventional automatic transmission shift control systems are provided with a solenoid valve controlled 3--2 timing valve for controlling a 3--2 shift down timing. The 3--2 timing valve is kept actuated or turned on for a time period set according to vehicle speeds. Such a time period is controlled by learning changes in speed of a turbine of the torque converter caused as a result of the 3--2 down-shift. Such an automatic transmission shift control system is known from, for instance, Japanese Unexamined Patent Publication No. 63-312,558.
The 3--2 timing valve of the automatic transmission shift control system described in the publication mentioned above is controlled in accordance with vehicle speeds, and the time period for which the 3--2 valve is kept opened or turned on is "learning-controlled" simply based on changes in speed of the turbine. In this automatic transmission shift control system, however, there is still a problem in that shift shock is not steady, due to varying rapidness and depression stroke of an accelerator pedal, even though the vehicle speed may be steady. If depression of the accelerator pedal, which is ordinarily made for a down-shift, is performed slowly, the output torque of the engine, and hence line pressure, increases following the slow depression of the accelerator pedal, so as to lock a 2--4 brake quickly as a result of actuating or turning on the 3--2 timing valve. Also, since the output torque of the engine can not increase sufficiently following quick depression of the accelerator pedal, the line pressure develops with a time lag after a down-shift demand occurs, so as to slow the locking of the 2--4 brake. Such a quick or slow depression of the accelerator pedal for a 3--2 down-shift causes the 2--4 brake to be locked at an improper timing, and shift shock is not reduced sufficiently during the down-shift.
In a conventional control system, in which the 3--2 timing valve is controlled according to vehicle speeds and the duration time learning control is performed for the 3--2 timing valve based on a change in turbine speed, it is also difficult to always keep an optimum learning control. Consequently, shift shock during a gear shift is not sufficiently prevented. This is because even if the vehicle travels at a constant speed, a boost in turbine speed differs according to the quickness of depression of an accelerator pedal and the stroke of depression of the accelerator pedal or the opening of a throttle valve. For the same reason, if the accelerator pedal is depressed faster or slower than is ordinary, a boost or decline in turbine speed is undesirably enhanced. Consequently, even for the same turbine speed, a boost in turbine speed may occasionally fall into or out of a standard range of speed changes established for the proper duration time learning control according to depression strokes of the accelerator pedal. This leads to an unstable judgement of a proper speed change. For example, an accelerator pedal may be operated so as to tend to boost the turbine speed after an acceleration operation is repeated, which tends to cause the speed of the turbine to cause the speed of the turbine to decline and, consequently, to make the learned duration time longer and longer. As a result, the turbine is undesirably boosted to an excessively high speed. Otherwise, if the accelerator pedal is operated so as to tend to reduce the turbine speed after an acceleration operation is repeated, which tends to cause the speed of the turbine to be easily boosted and, consequently, to make the learned duration time shorter and shorter, the turbine speed is undesirably reduced to an excessively low speed. In any case, heavy shift shocks occur.